The pH level isn't caused by fish poop or anything like that. If you're using tap water then that's where the pH level is coming from and you can't really change it. You can use chemicals but I highly suggest you don't. betta's are extremely adaptable and while that's not that high, it's really not going to hurt him at all. A stable pH level is what is good, not on that fluctuates everywhere! So 8.0 is totally fine.